In recent years, an attempt to reduce the weight of automobiles has been carried forward from the viewpoints of resource saving and energy saving. In order to reduce the weight of automobiles, a change in structure and a change in materials for various components constituting automobiles have been enthusiastically performed in the art. In particular, a change in the material from iron having high specific gravity to a light alloy, such as an aluminum alloy, has been performed because the reduction in weight can be simply achieved.
Components for which the material is changed from iron to the aluminum alloy range from engine components occupying about 20% of the weight of the vehicle to drive system components and suspension system components. For example, the transmission case has hitherto been made of iron. However, use of a light alloy for forming the transmission case has been studied for the above reason. The transmission case covers the outside of a transmission portion having a number of gears which serve to transmit the output from an engine to a drive shaft and change number of revolutions. A rear cover requires a bearing-holding hole for supporting the drive shaft and is formed in a small thickness because the mechanical strength of the body is not required to be very high.
An increase in performance and grade of automobiles in recent years has led to an ever-increasing demand for a reduction in vibration and noise level. In this case, the preparation of a transmission case containing a number of gears, which are rotated at a high speed, and a rear cover for the transmission case using an aluminum alloy poses a problem of noise derived from vibration. Specifically, in the case of the conventional transmission case made of iron, the above problem is not very serious because the damping capacity of the material per se is high, whereas use of the aluminum alloy instead of iron poses the above problem due to low damping capacity of the material per se (aluminum alloy).